Power tongs have been used extensively in the oil industry for many years to speed the process of making-up and breaking-apart connections between oilfield tubular goods during drilling and work-over operations. Such power tongs typically include a rotary mechanism for gripping the external surface of one threaded member, and then rotating that member to make or break the threaded connections. In some cases, a back-up tong is used to ensure that the lower threaded member remains stationary when the upper threaded member is rotated by the power tong.
A power tong may be of the closed throat type, wherein a 360.degree. rotatable ring is positioned about the connection, or the open throat type, wherein the rotatable ring includes a slot or throat. A closed throat tong ring is inherently structurally preferred, but the open throat ring desirably allows the tong to be laterally moved on or taken off a tubular connection. The present invention is primarily directed to an open throat power tong. Open-throat power tongs typically are repeatedly moved laterally on then off a pipe string each time a threaded connection is made up or broken out.
An open throat or partial rotary drive ring may typically be mounted with guide rollers for rotation with respect to the power tong frame about a rotation axis which extends through the power tong throughbore. The rotary drive ring may typically define camming surfaces on an interior surface of the rotary drive ring. A cage plate assembly is mounted for rotation with the rotary drive gear. One or more heads carried by the cage plate thus grasp the pipe upon relative motion between the cage plate assembly and one or more camming surfaces provided on the rotary drive ring. Once the cage plate assembly is in gripping engagement with the pipe, the drive ring is then rotated to simultaneously rotate the cage plate assembly and thus the pipe for the threading or unthreading operation.
Each year numerous power-tong related accidents are reported, typically involving injured fingers and hands. The large number of moving parts in the power tong operate with a torque that is practically impervious to an accidentally misplaced finger or hand. Even when the tong is in "neutral" and the gears slowly creep due to circulating hydraulic fluid, the forces are generally sufficient to sever a misplaced finger.
The hazardous nature of power tong operation is a problem that has long been recognized by the industry although satisfactory solutions have not been devised to significantly reduce or eliminate this condition. In an attempt to solve these problems, power tongs have been designed so that the operator must have both hands on control levers before the tong operates, however this solution does not protect those persons who must continually open and close the tongs so that they can be laterally moved on and off the tubular goods. Also, such dual lever concepts can be by passed, e.g., by two operators each having one hand on one control, or by mechanically interlocking the two levers. Various power tong designs have partial guards that provide protection with respect to some of the power tong's moving parts. However, presently available open throat power tongs do not provide shielding for all or substantially all of the tong moving parts. The problems associated with adequate tong shielding include recognition that the power tong practically must be able to operate on different diameter tubulars. Also, existing power tongs do not contain adequate safeguards to ensure that safely devices are properly in place prior to actuating the tong, and the hydraulic circuits used to drive the power tongs allow for high force movement of tong components even when no such movement is desired by the setting of the operator controls.
Thus, there has been a long felt need in the industry to provide an improved power tong designed to significantly reduce accidents while retaining efficient operation, that is adaptable to presently existing tongs, and that has a reasonable cost. Those persons skilled in the art will appreciate the present invention which provides solutions to these and other problems associated with power tongs.